Improvements in microelectronics have increased the demand for electrolytic cells and batteries that can be directly incorporated into electronic devices so as to produce a portable, finished package. As the improvements in microelectronics reduce the size of the electronic device, the space allotted to a power supply within such device has likewise decreased. It is therefore important to maximize the power-per-unit space that a battery cell can provide. One way to improve power-per-unit space is to reduce the size of the packaging or enclosure containing the electrolytic cell.
Traditionally, batteries, particularly small batteries, are packaged in metal cans (e.g., cylindrical batteries and button cells) or within molded plastic cases (e.g., SLI batteries). These types of enclosures and packages contain the active components of the battery, insulate them from the environment and provide overall mechanical strength. Such enclosures are dimensioned to be fairly rigid to withstand significant internal pressures. In this respect, in some types of batteries, internal gas buildup may occur resulting in relatively high internal pressures. These pressures must be contained within the enclosure to avoid explosion or violent venting. Both plastic and metal enclosures can provide the necessary structural strength to withstand most internal pressures, and do so in a volume efficient manner. Metal containers are most "volume efficient" in that relatively thin-walled metal containers can withstand significant pressures yet the volume of the container itself presents only a small portion of a battery's overall volume.
While metal and molded plastic containers are sturdy and volume efficient, they are somewhat shape restrictive. Typically, metal and molded plastic containers are most suitable and cost effective when traditional cylindrical shapes (especially for metal enclosures) or rectangular shapes are required. However, when a specific application or product requires a battery having an unusual shape, such as an extremely thin battery or a non-planar or non-prismatic battery, the assembly of a battery in an unusually shaped metal or plastic enclosure becomes more difficult.
For extremely thin or unusually shaped batteries, flexible packaging has found advantageous application. Flexible packaging can provide hermetic containment of the battery, and in addition, has several advantages over traditional rigid metal or plastic packaging. Foremost, flexible materials are lighter and they can conform more easily to the shape of the battery structure, thus providing easier and more cost effective manufacturing. Further, a flexible laminate package enables more efficient use of the space available within a device, in that such a package can assume a variety of shapes allowing the battery to be designed to accommodate the space restriction within a device. Still further, flexible packaging is less susceptible to a violent venting or catastrophic failure caused by a build-up of internal pressure as compared to rigid metal or plastic containers. In this respect, flexible packaging ruptures at a fraction of the pressure of rigid metal or plastic enclosures.
The advantages of flexible laminate packaging are particularly valuable for small, high-energy batteries. In general, the smaller the battery becomes in any one dimension, the greater is the contribution of the packaging to the overall weight and volume of the battery. With smaller batteries, the metal cans or molded plastic cases previously described decrease the measure of the "energy density" of such batteries. (As used herein, the term "energy density" shall refer to a weight measurement of the amount of cell material (in ounce/liter) within a particular volume of battery.) For these small, high-energy batteries, foil laminate packaging finds advantageous application by providing weight savings over metal cans and plastic cases. However, if the final package is not carefully configured, any potential weight and shape advantage of the flexible packaging may be lost, or the overall volume may even increase due to the packaging configuration.
The weight savings of a flexible laminate packaging may also be lost when packaging a non-aqueous battery, such as a high-energy lithium ion battery. If long storage and operating life are to be obtained for such batteries, the packaging or enclosure must provide and maintain a hermetic barrier that will prevent electrolytic solvents from escaping from the battery and water vapor and oxygen from penetrating into the battery. Such a barrier is typically provided by a layer of metal foil within the flexible laminate. In order to maintain the hermetic barrier, it is also important that the laminate be formed into a bag or package without significant stretching or deformation of the laminate that may tear or rupture the metal foil layer.
FIGS. 1-5 show a conventional package configuration formed of a flexible laminate for an electrolytic battery. Typically, the packaging is formed from a flexible laminate having a polymer layer for toughness, a metal layer to form a hermetic barrier and an interior adhesive layer. The packaging is typically formed by placing a rectangular electrolytic cell onto one side of a sheet of the flexible laminate. The cell is positioned such that a portion of each of the leads extending from the cell is positioned on the laminate, and a portion of the leads extends beyond the edge of the laminate. The other half of the laminate sheet is then folded over onto the battery to overlay onto the other side of the sheet, with the interior adhesive layer in contact with itself along three peripheral edges where the laminate extends beyond the cell. In this respect, the laminate sheet is dimensioned such that when folded over, it extends beyond three peripheral edges of the cell. Heat and pressure are applied to these three edges to form a seal about the periphery of the cell.
As can be seen in FIG. 1, the sealed regions of the laminate that extend about the three sides of the cell occupy a significant amount of space. If the finished battery is intended to fit into a rectangular cavity within an electronic device, these sealed edges must be folded onto the cell body. These sealed regions may be folded onto the cell body as illustrated in FIGS. 2 and 3, but the resulting edge and corner projections (as best seen in FIG. 5) includes a buildup wherein seven layers of the laminate are overlaid onto each other. This built-up region of overlapping layers of the laminate reduces the volume efficiency, i.e., the "energy density" of the battery.
Another problem associated with flexible packaging is forming a hermetic seal around the battery leads that extend through the packaging. Typically, such leads (conventionally referred to as "feedthroughs") are formed of strips of metal foil, such as copper or aluminum, and have a thickness approximately equal to that of the adhesive layer on the flexible laminate. Because the heat and pressure that can be applied to the laminate when forming the battery package (i.e., sealing the edges) must be limited so as not to tear the laminate during the sealing process, creating a complete and permanent seal about the feedthroughs is a major problem in flexible laminate packaging. FIG. 10 pictorially illustrates how voids may form along the edges of a feedthrough as a result of the limited heat and pressure that can be applied to the flexible laminate during a conventional sealing process. Such voids may allow air or moisture to penetrate the package or may allow electrolyte to escape from the package, both of which basically destroy the effectiveness of the battery. Excessive heat and pressure, on the other hand, can create the problem shown in FIG. 11, wherein the metal foil layer of the laminate is pressed near or into contact with the feedthrough, as the softened adhesive material is forced laterally by the pressure of a forming platen. Contact between the feedthrough and the metal foil can "short circuit" the battery. In this respect, both leads are typically pressed simultaneously by the same forming platen, and if the metal foil is pressed into contact with one feedthrough, it will generally also be pressed into contact with the other feedthrough. Even if the increased pressure does not create a short circuit (or tear the laminate), there is no certainty that the voids along the lateral edges of the feedthroughs will be closed.
The present invention overcomes these and other problems and provides an improved method of forming a hermetic package for a battery from a flexible laminate material, including an improved method of sealing feedthroughs.